The present invention relates generally to high speed networking transceivers and, more particularly to gigabit Ethernet transceivers having reduced power consumption, efficient clock domain partitioning and able to decode input symbols within a symbol period with a minimum of computational intensity.
In recent years, local area network (LAN) applications have become more and more prevalent as a means for providing local interconnect between personal computer systems, work stations and servers. Because of the breadth of its installed base, the 10BASE-T implementation of Ethernet remains the most pervasive if not the dominant, network technology for LANs. However, as the need to exchange information becomes more and more imperative, and as the scope and size of the information being exchanged increases, higher and higher speeds (greater bandwidth) are required from network interconnect technologies. Among the highspeed LAN technologies currently available, fast Ethernet, commonly termed 100BASE-T, has emerged as the clear technological choice. Fast Ethernet technology provides a smooth, non-disruptive evolution from the 10 megabit per second (Mbps) performance of 10BASE-T applications to the 100 Mbps performance of 100BASE-T. The growing use of 100BASE-T interconnections between servers and desktops is creating a definite need for an even higher speed network technology at the backbone and server level.
One of the more suitable solutions to this need has been proposed in the IEEE 802.3ab standard for gigabit Ethernet, also termed 1000BASE-T. Gigabit Ethernet is defined as able to provide 1 gigabit per second (Gbps) bandwidth in combination with the simplicity of an Ethernet architecture, at a lower cost than other technologies of comparable speed. Moreover, gigabit Ethernet offers a smooth, seamless upgrade path for present 10BASE-T or 100BASE-T Ethernet installations.
In order to obtain the requisite gigabit performance levels, gigabit Ethernet transceivers are interconnected with a multi-pair transmission channel architecture. In particular, transceivers are interconnected using four separate pairs of twisted Category-5 copper wires. Gigabit communication, in practice, involves the simultaneous, parallel transmission of information signals, with each signal conveying information at a rate of 250 megabits per second (Mb/s). Simultaneous, parallel transmission of four information signals over four twisted wire pairs poses substantial challenges to bidirectional communication transceivers, even though the data rate on any one wire pair is xe2x80x9conlyxe2x80x9d 250 Mbps.
In particular, the gigabit Ethernet standard requires that digital information being processed for transmission be symbolically represented in accordance with a five-level pulse amplitude modulation scheme (PAM-5) and encoded in accordance with an 8-state Trellis coding methodology. Coded information is then communicated over a multi-dimensional parallel transmission channel to a designated receiver, where the original information must be extracted (demodulated) from a multi-level signal. In gigabit Ethernet, it is important to note that it is the concatenation of signal samples received simultaneously on all four twisted pair lines of the channel that defines a symbol. Thus, demodulator/decoder architectures must be implemented with a degree of computational complexity that allows them to accommodate not only the xe2x80x9cstate widthxe2x80x9d of Trellis coded signals, but also the xe2x80x9cdimensional depthxe2x80x9d represented by the transmission channel.
Computational complexity is not the only challenge presented to modern gigabit capable communication devices. A perhaps greater challenge is that the complex computations required to process xe2x80x9cdeepxe2x80x9d and xe2x80x9cwidexe2x80x9d signal representations must be performed in an almost vanishingly small period of time. For example, in gigabit applications, each of the four-dimensional signal samples, formed by the four signals received simultaneously over the four twisted wire pairs, must be efficiently decoded within a particular allocated symbol time window of about 8 nanoseconds.
Successfully accomplishing the multitude of sequential processing operations required to decode gigabit signal samples within an 8 nanosecond window requires that the switching capabilities of the integrated circuit technology from which the transceiver is constructed be pushed to almost its fundamental limits. If performed in conventional fashion, sequential signal processing operations necessary for signal decoding and demodulation would result in a propagation delay through the logic circuits that would exceed the clock period, rendering the transceiver circuit non-functional. Fundamentally, then, the challenge imposed by timing constraints must be addressed if gigabit Ethernet is to retain its viability and achieve the same reputation for accurate and robust operation enjoyed by its 10BASE-T and 100BASE-T siblings.
In addition to the challenges imposed by decoding and demodulating multilevel signal samples, transceiver systems must also be able to deal with intersymbol interference (ISI) introduced by transmission channel artifacts as well as by modulation and pulse shaping components in the transmission path of a remote transceiver system. During the demodulation and decoding process of Trellis coded information, ISI components are introduced by either means must also be considered and compensated, further expanding the computational complexity and thus, system latency of the transceiver system. Without a transceiver system capable of efficient, high-speed signal decoding as well as simultaneous ISI compensation, gigabit Ethernet would likely not remain a viable concept.
In a Gigabit Ethernet communication system that conforms to the 1000BASE-T standard, gigabit transceivers are connected via Category 5 twisted pairs of copper cables. Cable responses vary drastically among different cables. Thus, the computations, and hence power consumption, required to compensate for noise (such as echo, near-end crosstalk, far-end crosstalk) will vary widely depending on the particular cable that is used.
In integrated circuit technology, power consumption is generally recognized as being a function of the switching (clock) speed of transistor elements making up the circuitry, as well as the number of component elements operating within a given time period. The more transistor elements operating at one time, and the higher the operational speed of the component circuitry, the higher the relative degree of power consumption for that circuit. This is particularly relevant in the case of Gigabit Ethernet, since all computational circuits are clocked at 125 Mhz (corresponding to 250 Mbps per twisted pair of cable), and the processing requirements of such circuits require rather large blocks of computational circuitry, particularly in the filter elements. Power consumption figures in the range of from about 4.5 Watts to about 6.0 Watts are not unreasonable when the speed and complexity of modern gigabit communication circuitry is considered.
Pertinent to an analysis of power consumption is the realization that power is dissipated, in integrated circuits, as heat. As power consumption increases, not only must the system be provided with a more robust power supply, but also with enhanced heat dissipation schemes, such as heat sinks (dissipation fins coupled to the IC package), cooling fans, increased interior volume for enhanced air flow, and the like. All of these dissipation schemes involve considerable additional manufacturing costs and an extended design cycle due to the need to plan for thermal considerations.
Prior high speed communication circuits have not adequately addressed these thermal considerations, because of the primary necessity of accommodating high data rates with a sufficient level of signal quality. Prior devices have, in effect, xe2x80x9chard wiredxe2x80x9d their processing capability, such that processing circuitry is always operative to maximize signal quality, whether that degree of processing is required or not. Where channel quality is high, full-filter-tap signal processing more often obeys the law of diminishing returns, with very small incremental noise margin gains recovered from the use of additional large blocks of active filter circuitry.
This trade-off between power consumption and signal quality has heretofore limited the options available to an integrated circuit communication system designer. If low power consumption is made a system requirement, the system typically exhibits poor noise margin or bit-error-rate performance. Conversely, if system performance is made the primary requirement, power consumption must fall where it may with the corresponding consequences to system cost and reliability.
Accordingly, there is a need for a high speed integrated circuit communication system design which is able to accommodate a wide variety of worst-case channel (cable) responses, while adaptively evaluating signal quality metrics in order that processing circuitry might be disabled, and power consumption might thereby be reduced, at any such time that the circuitry is not necessary to assure a given minimum level of signal quality.
Such a system should be able to adaptively determine and achieve the highest level of signal quality consistent with a given maximum power consumption specification. In addition, such a system should be able to adaptively determine and achieve the lowest level of power consumption consistent with a given minimum signal quality specification.
The present invention is a method and a system for providing an input signal from a multiple decision feedback equalizer to a decoder based on a tail value and a subset of coefficient values received from a decision-feedback equalizer. A set of pre-computed values based on the subset of coefficient values is generated. Each of the pre-computed values is combined with the tail value to generate a tentative sample. One of the tentative samples is selected as the input signal to the decoder.
In one aspect of the system, tentative samples are saturated and then stored in a set of registers before being outputted to a multiplexer which selects one of the tentative samples as the input signal to the decoder. This operation of storing the tentative samples in the registers before providing the tentative samples to the multiplexer facilitates high-speed operation by breaking up a critical path of computations into substantially balanced first and second portions, the first portion including computations in the decision-feedback equalizer and the multiple decision feedback equalizer, the second portion including computations in the decoder.
The present invention can be directed to a system and method for decoding and ISI compensating received signal samples, modulated for transmission in accordance with a multi-level alphabet, and encoded in accordance with a multi-state encoding scheme. Modulated and encoded signal samples are received and decoded in an integrated circuit receiver which includes a multi-state signal decoder. The multi-state signal decoder includes a symbol decoder adapted to receive a set of signal samples representing multi-state signals and evaluate the multi-state signals in accordance with the multi-level modulation alphabet and the multi-state encoding scheme. The symbol decoder outputs tentative decisions.
An ISI compensation circuit is configured to provide ISI compensated signal samples to the symbol decoder. The ISI compensation circuit is constructed of a single decision feedback equalizer, with the single decision feedback equalizer providing ISI compensated signal samples to the symbol decoder based on tentative decisions outputted by the symbol decoder.
In one aspect of the invention, a path memory module is coupled to the symbol decoder and receives decisions and error terms from the symbol decoder. The path memory module includes a plurality of sequential registers, with each corresponding to a respective one of consecutive time intervals. The registers store decisions corresponding to the respective ones of the states of the multi-state encoded signals. Decision circuitry selects a best decision from corresponding ones of the registers, with the best decision of a distal register defining a final decision. The best decision of an intermediate register defines a tentative decision which is output to the ISI compensation circuit.
The single decision feedback equalizer is configured as an FIR filter, and is characterized by a multiplicity of coefficients, subdivided into a set of high-order coefficients and a set of low-order coefficients. Tentative decisions from the path memory module are forced to the single decision feedback equalizer at various locations along the filter delay line and are combined with the high-order coefficients in order to define a partial ISI component. The partial ISI component is arithmetically combined with an input signal sample in order to generate a partially ISI compensated intermediate signal called tail signal.
Low-order coefficients from the single decision feedback equalizer are directed to a convolution engine wherein they are combined with values representing the levels of a multi-level modulation alphabet. The convolution engine outputs a multiplicity of signals, representing the convolution results, each of which are arithmetically combined with the tail signal to define a set of ISI compensated tentative signal samples.
In a particular aspect of the invention, the ISI compensated tentative signal samples are saturated and then stored in a set of registers before being outputted to a multiplexer circuit which selects one of the tentative signal samples as the input signal to the symbol decoder. Storing tentative signal samples in the set of registers before providing the tentative signal samples to the multiplexer, facilitates high-speed operation by breaking up a critical path of computations into substantially balanced first and second portions, the first portion including computation in the ISI compensation circuitry, including the single decision feedback equalizer and the multiple decision feedback equalizer, the second portion including computations in the symbol decoder.
In a further aspect of the present invention, symbol decoder circuitry is implemented as a Viterbi decoder, the Viterbi decoder computing path metrics for each of the N states of a Trellis code, and outputting decisions based on the path metrics. A path memory module is coupled to the Viterbi decoder for receiving decisions. The path memory module is implemented with a number of depth levels corresponding to consecutive time intervals. Each of the depth levels includes N registers for storing decisions corresponding to the N states of the trellis code. Each of the depth levels further includes a multiplexer for selecting a best decision from the corresponding N registers, the best decision at the last depth level defining the final decision, the best decisions at other selected depth levels defining tentative decisions.
In a particular aspect of the invention, tentative decisions are generated from the first three depth levels of the path memory module. These tentative decisions are forced to a single decision feedback equalizer to generate a partial ISI component based on the first three tentative decisions and a set of high-order coefficients. The partial ISI component is arithmetically combined with an input signal sample in order to define a partially ISI compensated tentative signal sample.
The first two coefficients of the single decision feedback equalizer are linearly combined with values representing the five levels of a PAM-5 symbol alphabet, thereby generating a set of 25 pre-computed values, each of which are arithmetically combined with the partial ISI compensated signal sample to develop a set of 25 samples, one of which is a fully ISI compensated signal sample and is chosen as the input to the symbol decoder.
The present invention is further directed to a system and method for decoding information signals modulated in accordance with a multi-level modulation scheme and encoded in accordance with a multi-state encoding scheme by computing a distance between a received word from a codeword included in a plurality of code-subsets. Codewords are formed from a concatenation of symbols from a multi-level alphabet, with the symbols selected from two disjoint symbol-subsets X and Y. A received word is represented by L inputs, with L representing the number of dimensions of a multi-dimensional communication channel. Each of the L inputs uniquely corresponds to one of the L dimensions. A set of 1-dimensional (1D) errors is produced from the L inputs, with each of the 1D errors representing a distance metric between a respective one of the L inputs and a symbol in one of the two disjoint symbol-subsets. 1D errors are combined in order to produce a set of L-dimensional errors such that each of the L-dimensional errors represents a distance between the received word and a nearest codeword in one of the code-subsets.
In one embodiment of the invention, each of the L inputs is sliced with respect to each of the two disjoint symbol-subsets X and Y in order to produce a set of X-based errors, a set of Y-based errors and corresponding sets of X-based and Y-based decisions. The sets of X-based and Y-based errors form the set of 1D errors, while the sets of X-based and Y-based decisions form a set of 1D decisions. Each of the X-based and Y-based decisions corresponds to a symbol, in a corresponding symbol subset, closest in distance (value) to one of the L inputs. Each of the 1D errors represents a distance metric between a corresponding 1D decision and the respective one of the L inputs.
In another embodiment of the invention, each of the L inputs are sliced with respect to each of the two disjoint symbol subsets X and Y in order to produce a set of 1D decisions. Each of the L inputs is further sliced with respect to a symbol-set including all of the symbols of the two disjoint symbol-subsets in order to produce a set of hard decisions. The X-based and Y-based 1D decisions are combined with a set of hard decisions in order to produce a set of 1D errors, with each of the 1D errors representing a distance metric between a corresponding 1D decision and a respective one of the L inputs.
In one embodiment of the present invention, 1-dimensional errors are combined in a first set of adders in order to produce a set of 2-dimensional errors. A second set of adders combines the 2-dimensional errors in order to produce intermediate L-dimensional errors, with the intermediate L-dimensional errors being arranged into pairs of errors such that the pairs of errors correspond one-to-one to the code-subsets. A minimum-select module determines a minimum for each of the pairs of errors. Once determined, the minima are defined as the L-dimensional errors.
The present invention is further directed to a method for dynamically regulating the power consumption of a high-speed integrated circuit which includes a multiplicity of processing blocks. A first metric and a second metric, which are respectively related to a first performance parameter and a second performance parameter of the integrated circuit, are defined. The first metric is set at a pre-defined value. Selected blocks of the multiplicity of processing blocks are disabled in accordance with a set of pre-determined patterns. The second metric is evaluated, while the disabling operation is being performed, to generate a range of values of the second metric. Each of the values corresponds to the pre-defined value of the first metric. A most desirable value of the second metric is determined from the range of values and is matched to a corresponding predetermined pattern. The integrated circuit is subsequently operated with selected processing blocks disabled in accordance with the matching pre-determined pattern.
In particular, the first and second performance parameters are distinct and are chosen from the parametric group consisting of power consumption and a signal quality figure of merit. The signal quality figure of merit is evaluated while selected blocks of the multiplicity of processing blocks are disabled. The set of selected blocks which give the lowest power consumption, when disabled, while at the same time maintaining an acceptable signal quality figure of merit at a pre-defined threshold level is maintained in a disabled condition while the integrated circuit is subsequently operated.
In one aspect of the present invention, reduced power dissipation is chosen as the most desirable metric to evaluate, while a signal quality figure of merit is accorded secondary consideration. Alternatively, a signal quality figure of merit is chosen as the most desirable metric to evaluate, while power dissipation is accorded a secondary consideration. In a further aspect of the present invention, both signal quality and power dissipation are accorded equal consideration with selective blocks of the multiplicity of processing blocks being disabled and the resultant signal quality and power dissipation figures of merit being evaluated so as to define a co-existing local maxima of signal quality with a local minima of power dissipation.
In one particular embodiment, the present invention may be characterized as a method for dynamically regulating the power consumption of a communication system which includes at least a first module. The first module can be any circuit block, not necessarily a signal processing block. Power regulation proceeds by specifying a power dissipation value and an error value. An information error metric and a power metric is computed. Activation and deactivation of at least a portion of the first module of the communication system is controlled according to a particular criterion. The criterion is based on at least one of the information error metric, the power metric, the specified error and the specified power, to regulate at least one of the information metric and the power metric.
In particular, at least a portion of the first module is activated if the information error metric is greater than the specified error and the first module portion is deactivated if the information error metric is less than the specified error. In an additional aspect of the invention, the first module portion is activated if the information error metric is greater than the specified error and the power metric is smaller than the specified power. The first module portion is deactivated if the information error metric is smaller than the specified error or the power metric is greater than the specified power. In yet a further aspect of the invention, the first module portion is activated if the information error metric is greater than the specified error and is deactivated if the information error metric is smaller than a target value, the target value being smaller than the specified error. In yet another aspect of the invention, the first module portion is activated if the information error metric is greater than the specified error and the power metric is smaller than the specified power. The first module portion is deactivated if the information error metric is smaller than a target value, the target value being smaller than the specified error, or the power metric is greater than the specified power.
Advantageously, the information error metric is related to a bit error rate of the communication system and the information error metric is a measure of performance degradation in the communication system caused by deactivation of the portion of the first module. Where the module is a filter which includes a set of taps, with each of the taps including a filter coefficient, the information error metric is a measure of performance degradation of a transceiver caused by operation of the filter.
Power dissipation reduction is implemented by deactivating subsets of taps which make up the filter, until such time as performance degradation caused by the truncated filter reaches a pre-determined threshold level.
The present invention further provides a method for reducing system performance degradation caused by switching noise in a system which includes a set of subsystems. Each of the subsystems includes an analog section and a digital section. Each of the analog sections operates in accordance with a corresponding one of a set of sampling clock signals which are synchronous in frequency. The digital sections operate in accordance with a receive clock signal. The receive clock signal is generated such that it is synchronous in frequency with the sampling clock signals and has a phase offset with respect to one of the sampling clock signals. This phase offset is adjusted such that system performance degradation due to coupling of switching noise from the digital sections to the analog sections is substantially minimized.